Many different techniques have been proposed over the years for removing contaminants from hazardous waste sites, such as contaminated soil, all of which have suffered from one or more disadvantages which have made their use either technically or economically impractical. As used herein, soil can generally be understood to mean an earth-like medium having porosities ranging from a very densely packed clay medium to a relatively loosely packed medium, such as loosely packed sand.
The excavation and subsequent treatment of contaminated soil, for example, by soil washing or incineration, is a costly technique and may expose workers using such process to health risks. Moreover, in the case of soil washing, the procedure may not extract all of the contaminants attached to the clay or silt components of the soil, while in the case of incineration, a site pollution problem may be replaced by the creation of an air pollution problem.
In situ collection and injection remediation techniques have also been proposed. Collection techniques, such as the collection of a contaminant plume by pumping and/or drains, often suffer from dilution by surrounding ground water during collection, thus increasing the pumping and treatment costs. Further, effective control of the direction of the flow is generally not possible due to soil heterogeneity and cracks. Injection techniques, such as by using chemicals or biological agents injected in situ into the soil to detoxify the wastes, suffer from the difficulty of achieving a uniform distribution of the detoxifying materials throughout the soil. Moreover, both collection and injection techniques based on the use of pressure driven liquid flows may be impossible to use in soils having low hydraulic permeability so that their use is generally limited to relatively high permeability soils, e.g., relatively sandy soils. Moreover, when using presently proposed in situ remediation methods, including high pressure soil flushing, vacuum or steam extraction, or radio frequency volatilization, many contaminant materials, and particularly heavy metals, cannot be removed because of the strong attachment forces which bind the metals to the soil particles.
Electroosmosis has been proposed for the dewatering and consolidation of clays or other soils to provide soil stabilization, such as for construction purposes. As used herein, electroosmosis is defined as the process of moving a liquid through a porous material by the application of an electric field. In accordance therewith voltage gradients are established in the soil and the water therein is thereby caused to migrate toward and accumulate at or near one of the electrodes which are used to create the electric field therein, the accumulated water therein being removed therefrom, as by pumping.
However, efforts to apply electrokinetics in general, and electroosmosis techniques in particular, to the removal of contaminants from a waste site, are relatively recent. The movement of metal ions in fully dissociated salts within samples of saturated sands and soils have been studied by Hammett in "A Study of the Processes Involved in the Electro-Reclamation of Contaminated Soils," R. Hammett, M. Sc. Thesis in Pollution and Environmental Control, Manchester University, Manchester, ENGLAND, October, 1980. For his particular conditions, the waste movement appeared to be governed primarily by ionic migration and not by electroosmosis. Limited studies of the use of electroosmosis in clay saturated with an organic acid contaminant have also been made by Renaud and Probstein in Renaud, P. C. and Probstein, R. F. "Electroosmotic Control of Hazardous Wastes" Physicol Chemical Hydrodynamics Vol. 9, No. 1/2, pp. 345-360, 1987. The studies show that electroosmosis techniques might be useful in hazardous waste sites, particulary where it is necessary to control the direction of flow and where the site has relatively low hydraulic permeability where hydraulic removal techniques become ineffective. While electroosmosis has shown some promise as a possible technique, no effective implementation of a system using electroosmosis has yet been specifically proposed or demonstrated.
Because of the many possible advantages that electroosmosis techniques might bring to the problem of in situ contaminant removal at hazardous waste sites, it is desirable that an effective and reliable method and system be devised using such technique.